I started last year at market and the leaf lettuce and butterhead types went over really well. I sold some cos as baby heads, but after the baby stage they bolted. So, I would stay away from those unless offering babies. Also, at my market the curly endive didn't go over well. Sweat lettuce was what everybody wanted. But maybe your market is different? Good luck!
Patty

As a small and new-from-scratch grower (2 acres), too, with a couple of seasons of experience, I agree with Patty_WI about leaf and butterhead being great, easy basics, and cos/romaine being harder to grow to maturity. Romaine and also crisphead (the crunchy head lettuce in supermarkets, for some people, that's all they know and want) take more time, attention, water, right weather. Harvesting baby and small leaves, and faster growing leaf lettuces in general, has been no problem, without dedicated irrigation, right through summer in variable weather.

Also, of course, there's mesuclun/baby leaf salad mix. It uses up more seed, but it goes for more, grows fast and easy, and probably covers a good percentage of your salad customers who'd otherwise buy other lettuce. You can also cut it a bit later with bigger leaves and it's a...salad mix. (I ended up growing quite a lot of all-lettuce mesclun, because flea beetles go straight to the brassicas -- mustards, kale, etc -- that're otherwise great in the mix, they pepper them with little holes if you don't use row cover from the start (unless you spray chemicals, yuck), so more work.)

As things evolve, you come up with great ways to grow just about all lettuces anytime, in any set-up, but for practical small-scale starting out tips, this is probably safe. But do plant test beds of EVERYTHING (just get a packet of each), 'cause you can't buy back the experience each year can bring!!

Lettuce is definitely climate specific. The further you get away from 50-60 degrees the harder it gets. Close to 90 and yer taking it to the limit. The great part is, though, that you get two chances, spring and fall.

In my experience, the fastest bolting lettuce is Black seeded Simpson. HOWEVER, it is also the fastest grower, makes a good loosehead, and is green as it can be. I wouldn't include it in mixes, because it will dominate everything else, but will always grow it.

I've been fooling around with oak leaf for two years and am giving up on it. It's really overrated IMHO.

I have been happy with 'rocky top mix' from Baker Creek, but haven't a clue what I'm picking.

This year I'm trying red sails and salad bowl as late lettuce.

I've had good luck with some romaine(parris cos?) but planted it very early.

I direct seed broadcast, then thin to a fairly dense planting in the bed, maybe 6". Usually I can then pick every other one at smaller stage, and come back as the rest mature.

Year before last I had great luck with batavian endive. I planted it broadcast in late summer and never thinned it. That seemed to keep it from spreading out becoming curly and it just grew some outrageous flat leaves, very nice, almost blanched due to the crowding. I treated it as a cutting lettuce and got three cuts till frost. People really liked it and the close planting meant the leaves didn't get splashed. Last year I couldn't get the seed, and regretted it.

I do intend to do more transplanted lettuce this year, as 'catch crops' between other longer-season plants(okra, squash, peppers) before they get too big.